How CDN Technology Accelerates Web Delivery: Fundamentals and Mechanics

What is CDN

CDN stands for Content Delivery Network. It adds a caching layer to the existing Internet, publishing website content to edge nodes closest to users so that they can retrieve the content locally, improving response speed and alleviating bandwidth and traffic bottlenecks.

In simple terms, CDN caches resources from the origin server on globally distributed nodes; when a user requests a resource, the nearest node returns the cached copy, avoiding a round‑trip to the origin server and reducing network congestion.

Solves the "first mile" problem on the server side

Mitigates bottlenecks between different ISPs

Reduces provincial outbound bandwidth pressure

Alleviates backbone network load

Optimizes distribution of hot content on the web

CDN Working Principle

Traditional Access Process

When a website does not use CDN caching, the access flow is:

User enters the domain name; the operating system queries the local DNS for the IP address.

Local DNS queries the root DNS for the domain's authoritative server (assuming the local cache has expired).

Root DNS returns the authoritative DNS record to the local DNS.

Local DNS queries the authoritative DNS for the domain's IP address.

The authoritative DNS responds with the IP record.

Local DNS returns the IP address to the user.

The user connects to the origin server using the IP address.

The origin server responds and sends the content back to the client.

CDN Access Process

With CDN caching, the flow changes to:

User enters the domain name; the operating system queries the local DNS for the IP.

Local DNS queries the root DNS for the authoritative server (cache assumed expired).

Root DNS returns the authoritative DNS record.

Local DNS obtains the record and queries the authoritative DNS for the IP.

The authoritative DNS returns a CNAME record.

Local DNS queries the smart routing DNS for the IP of the nearest CDN node.

Smart routing DNS selects the optimal CDN node based on algorithms and policies and returns its IP.

Local DNS returns this IP to the user.

The user connects to the CDN node.

The CDN node serves the cached content to the client (or fetches from origin if not cached).

By using DNS to transparently direct users to the nearest cache server, CDN provides seamless acceleration without requiring any client configuration.

Components of a CDN Network

Each CDN node functions like a web server placed near end users. Through DNS takeover, user requests are automatically routed to the nearest node, where the CDN server responds just like the origin server but with lower latency.

The CDN layer sits between the user side and the origin server.

Key components include:

Intelligent routing DNS (e.g., F5 3DNS) that selects the closest node based on predefined strategies and node health.

Load‑balancing devices (e.g., LVS, F5 BIG‑IP).

Cache servers (e.g., Squid).

Shared storage.

Glossary

CNAME Record

A CNAME (Canonical Name) record maps one domain name to another. When DNS looks up the left‑hand name, it follows the CNAME to the right‑hand name until it reaches an A or PTR record.

Example: you have resources at

docs.example.com

and want

documents.example.com

to point to the same resources. Adding a CNAME record that maps

documents.example.com

to

docs.example.com

makes all requests to the former resolve to the latter.

CNAME Domain

When you enable CDN for a domain, the CDN provider gives you a CNAME domain. You must add a CNAME record in your DNS that points your own acceleration domain to this CNAME domain so that all requests are routed to CDN nodes.

DNS

DNS (Domain Name System) translates human‑readable domain names into IP addresses. It is essential for routing user requests to the appropriate server. Common DNS providers include Alibaba Cloud DNS, DNSPod, AWS Route 53, Cloudflare, etc.

Origin Host

The origin host determines which specific site on the origin server the CDN should request when fetching content.

Example 1: Origin server domain is www.a.com , origin host is www.b.com . The actual origin request goes to the IP resolved for www.a.com but accesses the site www.b.com on that host. Example 2: Origin server IP is 1.1.1.1 , origin host is www.b.com . The request goes to 1.1.1.1 and serves the site www.b.com on that machine.

Protocol Origin

Protocol origin means the CDN uses the same protocol (HTTP or HTTPS) for fetching content from the origin as the client used to request it. If a client requests via HTTPS, the CDN node will also use HTTPS to retrieve the resource when it is not cached.